Friday, December 23, 2011

Thorotrast (Thorium dioxide) was used as an intravenous contrast agent during World War II. It is taken up by the reticuloendothelial system and is slowly excreted by the kidneys.

Radiographs reveal increased density of a small or normal-sized (depending on how long ago the injection was), granular spleen with a fine mottled appearance. The splenic capsule is not abnormally dense. The liver may also be denser, but this is not as pronounced on radiographs.

On CT scan, the liver and abdominal lymph nodes have slightly higher attenuation, although not to the extent of that of the spleen. The Thorotrast deposits are not seen on conventional MRI or on ultrasound.

The problem with Thorotrast is that it is an alpha-emitter and can cause liver tumors (intrahepatic cholangiocarcinoma and angiosarcoma), decades after injection.

The main differential consideration is gold deposition from prolonged treatment of rheumatoid arthritis, which can look identical across all imaging modalities discussed above. Miliary calcification from miliary tuberculosis or a vascular disorder that results in multiple calcifications can be similar on radiographs, but will have imaging characteristics of calcium on ultrasound and MRI. In addition, on radiography, miliary calcifications are generally coarser in appearance, larger, and fewer in number. Some people mention capsular calcifications and calcifications in splenic cysts in the differential, but these are pretty easy to differentiate from thorotrast spleen.

The image above shows a patient with a history of Thorotrast injection. The chest radiograph shows fine granular densities within the spleen. The CT image with bone windows show the granularity as well. The liver on this non-contrast study has a higher attenuation than normal (70 HU, compared to about 55 HU for normal). The ultrasound shows a small spleen, but is otherwise normal.

Special thanks to Dr. Tommaso Bartalena for adding gold deposition to the differential diagnosis.